Office 16 XLSM Workbooks saved in Office 365 are 20-30% smaller?

Have just upgraded (?) to Office 365 and after saving Office 16 generated XLSM workbooks, the saved workbooks, still identified as XLSM, are significantly smaller, from 150KB to 110KB, or 2569KB to 2112KB. Is the save algorithm / compression different for a XLSM file in O365?

From internet.
"XLS vs . XLSX: File Size. The Office XML Format uses ZIP and compression technologies for data storage. So, the Office XML format based XLSX has smaller file size than the traditional binary based XLS.28 de jan. de 2023"